The supply of electrical power to the various devices of an elevator system usually occurs during normal operation of the elevator from an electricity network. In connection with a malfunction of the electricity network, such as an electricity outage or a voltage reduction, the elevator system is switched to reserve power mode, in which case electrical power is supplied to the elevator system from a reserve power source, such as from a reserve power generator, from a battery, et cetera. The operation of the elevator system can continue without interruption also in reserve power mode during a malfunction of the electricity network, if the reserve power source is dimensioned to supply electrical power according to the requirement of normal operation of the elevator. Normal operation of the elevator can also be discontinued in reserve power mode; however, also in this case electrical power must be supplied to selected points, such as to the emergency lighting of the elevator car.
The capacity of the reserve power source, such as of the generator or battery, needed in the reserve power mode of an elevator system might grow to be high, especially if the reserve power source must be dimensioned to supply electrical power during a long-lasting malfunction of the electricity network. Since, on the other hand, in elevator systems, both in elevator systems with a machine room and those without a machine room, the space requirement of a reserve power source may form a problem, it is endeavored to dimension the reserve power source to be as small as possible in its capacity. These two mutually conflicting requirements might result in, inter alia, overloading of the reserve power source or a breakdown of operation.